Bio molecules such as proteins very vary in size and shape and thus there are thousands to tens of thousands of kinds of proteins in accordance with the sizes and shapes.
The protein is a polymer in which many kinds of amino acids are arranged. The amino acids include an amino group (—NH2) and a carboxyl group (—COOH). That is, one protein molecule is formed of many kinds of amino acids and thus has a zwitterions characteristic having not only cations but also anions. A property of the protein varies depending on an amount of net charges generated by coupling and separation of proton ions (H+).
There is a point where a sum of total charges becomes 0 in a bio molecule having many cations and anions. This point is referred to as an isoelectric point (pI). That is, the isoelectric point indicates a pH value in case where a net charge existing in the protein molecule becomes 0.
Accordingly, it is very important to find a pI value by measuring the pH value that the net charge in the protein molecule becomes 0. The pH value plays a major role in understanding of a property of the protein molecule.
However, typical isoelectric point measuring methods such as an isoelectric focusing method and the like cannot effectively measure the isoelectric point of the bio molecule, cannot attain an accurate value, and have poor reproducibility. In addition, the typical isoelectric point measuring methods have some troubles measuring the isoelectric points of proteins having a high hydrophobic property, proteins having many basic groups, or proteins that are very big or small.